Hi Rikki,
I have two ideas for you. One is very easy and the other takes a bit of time.

EASY Play the last eight bars of your song without a rhythm. You might want to go into Parts Setting and change the default Electric Bass setting to something like Mellow Ensemble that will sound as long as you hold the chord and place a suitable sound in the left hand part. In addition you can play the chord progression and take some liberties with the melody. When you are ready for the song, just turn on the beat.

NOT AS EASY Even with all the new pressure MIDI files still abound. Finding a song with the right intro or ending will take some hunting. When you find it just do a Sequencer to Composer transfer - up to 16 measures. Don’t forget to transpose into the Key of C. I like to use this method just to get the drum part, which I find the hardest thing to do, then play the other parts into the Composer.

Have at it,
Frank